U.S. Pat. No. 4,905,231, issued to W. F. Leung et al. on Feb. 27, 1990, discloses a multimedia communication system using a single packet switching network for the various media based on what is referred to as a multimedia virtual circuit. A virtual circuit is a packet-switched communication path between two endpoints. A network call setup procedure establishes a virtual circuit and an associated virtual circuit identifier based on a destination address. To route successive packets, the network needs only the virtual circuit identifier. In a multimedia virtual circuit, a virtual circuit is divided into multiple virtual channels at the workstations. Each channel represents a different information medium and has a separate channel identifier; the composite multimedia virtual circuit represents the resulting multimedia call. As the various multimedia call sources generate traffic, the workstation multiplexes the packetized traffic onto a single network virtual circuit. The destination workstation demultiplexes this traffic back into multiple channels, which it routes to a telephone, speaker, file, or other destination. This process preserves the temporal ordering of the information streams so that in a voice and video call, for example, the voice corresponds to the mouth movement in the video picture. The multimedia virtual circuit also provides a simple way to let a multimedia call destinatio associate the multiple media. A single incoming call notification arrives from the network to announce a call. The workstations then exchange signaling information over a virtual channel of the multimedia virtual circuit to set up all the necessary channels and to route them to the appropriate devices.
The Rapport multimedia conferencing system, disclosed in the S. R. Ahuja et al. article, "The Rapport Multimedia Conferencing System," Proceedings of Conference on Office Information Systems, March 1988, supports interactive, real-time distributed conferences among two or more people. Executing on personal workstations interconnected by separate data and voice networks, the Rapport system provides basic mechanisms to create, manage, and terminate conferences. The system provides an environment in which many types of meetings can take place, including telephone conversations, discussion among colleagues, and lectures. Existing workstation programs can be used during a conference to produce and edit data and displays for the conferees. Rapport is designed to help people emulate face-to-face conferences as closely as possible with their workstations. However, the reliance on separate networks for the different media (data and voice) substantially complicates the control of conference calls. Although concurrent participation in many conferences is possible, a user is only able to communicate on one call at a time. A Rapport system user may participate in many conferences concurrently by switching among contexts at the click of a mouse button. This is equivalent to being able to walk in and out of several meeting rooms (and one's office) instantly. It is anticipated that this capability will encourage users to keep many conferences active for long periods of time in much the same fashion as the use of screen windows allows one to keep many programs and files active with the present data networks. One such long-lived conference might be an intercom connection between a manager and a secretary. Others might be among the collaborators in a design project or the authors of a paper. It is anticipated that once the capability for multiple concurrent calls is provided, it will be useful to merge and split such calls. For example, the manager may ask the secretary to join a design project conference from time to time to assist the project team. The effective control of a plurality of multimedia calls poses a technical problem, particularly in the Rapport system arrangement comprising several separate networks, but also for the system of the above-referenced U.S. Pat. No. 4,905,231 employing a single network for all media.